A typical conveyor for conveying relatively small articles includes a frame having a pair of spaced parallel side rails and a conveyor belt is trained over spindles that are journaled in the side rails and are located at the ends of the conveyor frame. In certain installations, a drive unit, including an electric or hydraulic motor, is mounted on one of the side rails of the frame and is connected through a speed reducing transmission to one of the spindles to drive the belt. In other installations, the drive unit is located beneath the lower run of the conveyor belt and the belt is trained over a drive spindle in the drive unit. The drive spindle is connected via a timing belt or chain drive to the drive shaft of the motor of the drive unit.
The typical conveyor belt requires a belt tensioning mechanism to achieve proper tension on the belt and maintain frictional engagement with the drive spindle. A common form of belt tensioning mechanism is connected to the bearing blocks that journal a spindle at the end of the conveyor and the tensioning mechanism is constructed to move the bearing blocks and spindle outwardly to increase the tension on the belt. With a drive unit that is mounted beneath the lower run of the conveyor, a belt tensioning spindle is frequently incorporated in the drive unit.
Conveyor belts are typically designed with a 1% maximum stretch. This means that if a conveyor is 100 feet long, the belt will be approximately 200 feet long so that the 1% maximum stretch would be approximately 2 feet. Therefore, it is important that the belt tensioning system associated with the conveyor be adequate to accommodate the maximum possible stretch of the belt.
The driving traction to drive the belt is dependent not only on the belt tension but also on the degree of wrap of the belt around the drive spindle or other drive member. Therefore, it is important to design a drive system to obtain maximum wrap in order to achieve the desired driving traction.